During processing of the metal layers in a semiconductor structure, a phenomenon known as "metal notching" occurs for a high density pattern. Metal notching results in a line within a metal pattern that has a less than expected width due to inherent characteristics of the underlying structure. This results from the photoresist pattern being reduced in width below the expected pattern during developing thereof. Subsequent etching results in a smaller line within the metal pattern than was expected.
The reduction in the line width due to metal notching is a function of the surface reflectance of the underlying metal layer and the topology of the structure underlying the metal layer. Typically, the metal layer is a conformal layer that has very small peaks and valleys. When a portion of a patterned line lies within one of the valleys, it is necessary to deposit and pattern the photoresist in the valley to define this metal line. However, when developing the photoresist, the impinging light rays can be refracted towards the vertical sides of the pattern, thus reducing the width of the photoresist pattern. Subsequent etching of the metal layer with this pattern results in thinner than expected line widths. Typically, these valleys in the metal layer occur between underlying polycrystalline silicon lines which were fabricated in an earlier process step.
As surface densities increase and line widths decrease, this metal notching phenomenon becomes significantly disadvantageous. It is therefore necessary to develop a process whereby the width of the photoresist pattern is not reduced as a result of reflections from the surface of the metal layer.